The likelihood of risky behaviour by pregnant women was linked to age, socio-economic status and where they lived, the study found.

It sparked concerns women were still not getting the message about the health effects on their unborn babies.

Of women who gave birth in 2003-04, nearly 20 per cent aged under 25 admitted drinking alcohol while pregnant and 37 per cent smoked.

In the 30 and above age group, about 44 per cent of mothers drank alcohol and nearly 10 per cent smoked.

The report, based on a Longitudinal Study of Australian Children tracking more than 10,000 children since 2004, did not ask how much women drank or how often.

Dr Ben Edwards, the institute's study manager, said alcohol use in pregnant women was worst among the affluent, while those struggling financially were more likely to smoke, have high blood pressure, stress, anxiety or depression.

Children of poorer mums were also more likely to be born pre-term and have a low birth weight while more older mums suffered diabetes during pregnancy.

"The one in five who are smoking may need more help and assistance in reducing the amount they smoke and giving up before they become pregnant," he said.

"It seems younger women are starting to get the message about drinking but for affluent women, my feeling is that it is more of a social drink for some."

Women may also be confused by changes in public health warnings about how much alcohol is safe, he said.

The government now recommends zero intake while pregnant.

Heavy alcohol use is known to be harmful for the baby including risks of birth defects and miscarriage, but there is less known about the dangers of low to moderate alcohol consumption.

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